Will pruning our tall thin crape myrtles cause them to bush out?

Not really. The trees are growing tall and thin because they are looking for light. UT Extension Horticulture Specialist Celeste Scott says even if they are pruned, they will continue to stretch for the sun. She recommends growing them in a place where they will get more light, preferably full sun.
​
Related Videos:
Pruning (Not Murdering) Crape Myrtles
Crape Myrtles

Related Resources:
Crape Myrtle Pruning
Pruning Crapemyrtles
Crapemyrtle Pruning